Missing FIELDWIDTHS splitting mode #123

gawk supports splitting fixed-width fields with this variable. All the variable trapping for tracing modes, etc. has already been plumbed through. The only missing thing should be the implementation itself, but the docs confuse me a bit, and I'd be more comfortable with another pair of eyes taking a look.

Sorry for the delay, I had a deployment at work these last couple of weeks and couldn't get the time to put this together.

https://www.gnu.org/software/gawk/manual/html_node/Fixed-width-data.html
https://www.gnu.org/software/gawk/manual/html_node/Skipping-intervening.html
https://www.gnu.org/software/gawk/manual/html_node/Allowing-trailing-data.html
https://www.gnu.org/software/gawk/manual/html_node/Fields-with-fixed-data.html

Relevant files/lines: